Add placeholder for simSwapTexture in Unity for compilation

[rajat2004]

WIP for now, even though it compiles, starting Unity project gives errors
Even though the branch name says Windows, only works for Linux currently, still fails in Windows with linking error

WorldSimApi.obj : error LNK2001: unresolved external symbol "public: virtual class std::unique_ptr<class std::vector<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::allocator<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > >,struct std::default_delete<class std::vector<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::allocator<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > > __cdecl WorldSimApi::swapTextures(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,int,int,int)" (?swapTextures@WorldSimApi@@UEAA?AV?$unique_ptr@V?$vector@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V?$allocator@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@U?$default_delete@V?$vector@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V?$allocator@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@@2@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@3@HHH@Z) [C:\Users\travis\build\rajat2004\AirSim\Unity\AirLibWrapper\AirsimWrapper\AirsimWrapper.vcxproj]

C:\Users\travis\build\rajat2004\AirSim\Unity\AirLibWrapper\x64\Release\AirsimWrapper.dll : fatal error LNK1120: 1 unresolved externals [C:\Users\travis\build\rajat2004\AirSim\Unity\AirLibWrapper\AirsimWrapper\AirsimWrapper.vcxproj]

Caught while looking at #2356

[rajat2004]

Superseded by #2514